FinFET Varactor with Low Threshold Voltage and Method of Making the Same

ABSTRACT

Disclosed is a FinFET varactor with low threshold voltage and methods of making the same. A disclosed method includes receiving a semiconductor layer over a substrate and having channel, source, and drain regions. The method includes forming a well in the semiconductor layer to have a first dopant, and implanting a second dopant into the well. The first and second dopants are of opposite doping types. A first portion of the well has a higher concentration of the second dopant than the first dopant. A second portion of the well under the first portion has a higher concentration of the first dopant than the second dopant. The method further includes forming a gate stack over the channel region, and forming source and drain features in the source and drain regions. The first portion of the well electrically connects the source and drain features.

PRIORITY

This claims the benefits of U.S. Prov. App. Ser. No. 62/329,928, entitled “FinFET Varactor with Low Threshold Voltage and Method of Making the Same,” filed Apr. 29, 2016, herein incorporated by reference in its entirety.

BACKGROUND

MOS (metal-oxide-semiconductor) varactors are semiconductor devices that have a capacitance varying as a function of an applied voltage. Varactors are often used as tuning elements in circuits such as voltage controlled oscillators (VCOs), parameter amplifiers, phase shifters, phase locked loops (PLLs), and other tunable circuits. For example, by varying a voltage applied to a varactor, the frequency of operation of an associated VCO can be adjusted. Tunability, linearity, and quality factor are among the important characteristics of an MOS varactor.

As the semiconductor industry strives for higher device density, higher performance, and lower power, problems involving MOS varactor fabrication and design have been encountered, particularly with FinFET varactors in advanced process nodes such as 16 nanometer (nm) or smaller. For example, it has been found that FinFET varactors sometimes do not provide good tunability and linearity for ultra-low power circuits. For example, a linear tuning region of a FinFET varactor may be centered on a gate voltage (Vg) greater than 0 (zero) volts such as 0.3V, and may become almost un-tunable when Vg swings near 0V. That means a VCO circuit needs to provide a Vg biasing near 0.3V, while an ideal Vg biasing is near 0V for achieving ultra-low power consumption in the circuit. Accordingly, improvements in FinFET varactor design and fabrication are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 illustrates a VCO circuit having a MOS varactor constructed according to aspects of the present disclosure.

FIG. 2 illustrates a FinFET varactor according to aspects of the present disclosure.

FIGS. 3A and 3B show fragmentary schematic views of the FinFET varactor of FIG. 2, in accordance with some embodiments.

FIG. 4 shows a flow diagram of a method of forming a device having a FinFET varactor, according to various aspects of the present disclosure.

FIGS. 5A, 5B, 6A, 6B, 7, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, and 13B are cross-sectional or top views of forming a FinFET device according to the method of FIG. 4, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The present disclosure is generally related to MOS varactors, and more particularly to FinFET varactors having a low threshold voltage (Vt) and methods of making the same. FinFET varactors having a low Vt, such as a Vt near 0V, are greatly desired for ultra-low power applications. For example, they can be used in today's mobile devices such as smart phones and tablets where users demand ultra-low power consumption. However, the provided subject matter can be applied to other types of varactors, such as varactors formed with planar transistors instead of FinFETs, as well as varactors formed with other types of multi-gate transistors in addition to FinFETs.

FIG. 1 shows a VCO circuit (or design) 100 which may benefit from aspects of the present disclosure. Referring to FIG. 1, the VCO 100 includes a FinFET varactor C2 for tuning an operation frequency of the VCO 100. The FinFET varactor C2 is constructed according to an embodiment of the present disclosure, which will be described in details later. The VCO 100 further includes PMOSFETs (p-type MOS field-effect transistors) PM1 and PM2, NMOSFET (n-type MOSFETs) NM1 and NM2, another capacitor C1, an inductor L1, terminals connected to VDD (positive power supply) and GND (ground), and various output terminals Vtune, Voutn, and Voutp. FIG. 1 also illustrates a C-V curve 102 of the FinFET varactor C2 under certain working condition. A near-linear tuning range 104 of the FinFET varactor C2 is centered on a Vg that is at or close to 0V, such as ranging from 0 to 0.1V. That means a biasing voltage applied to the FinFET varactor C2 may be set to 0V or near 0V, which greatly reduces the power consumption of the VCO circuit 100.

FIG. 2 illustrates a fragmentary perspective view of a FinFET varactor 200, such as the FinFET varactor C2 of FIG. 1, according to various aspects of the present disclosure. Referring to FIG. 2, the FinFET varactor 200 includes a substrate 202, an isolation feature 206 and a fin 204 protruding out of the substrate 202 and through the isolation feature 206. The FinFET varactor 200 further includes a gate stack 208 having a gate dielectric layer 212 and a gate electrode 210. The gate electrode 210 may include one or more layers such as a work function metal layer, a metal barrier layer, a metal fill layer, and so on. The work function metal layer may be a p-type or an n-type work function layer. The p-type work function layer comprises a metal with a sufficiently large effective work function, selected from but not restricted to the group of titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof. The n-type work function layer comprises a metal with sufficiently low effective work function, selected from but not restricted to the group of titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), or combinations thereof. The metal fill layer may include aluminum (Al), tungsten (W), cobalt (Co), copper (Cu), and/or other suitable materials.

The gate dielectric layer 212 may comprise a high-k dielectric material such as hafnium oxide (HfO₂), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitable metal-oxides, or combinations thereof. The FinFET varactor 200 may further include an interfacial layer between the fin 204 and the gate dielectric layer 212, which may include a dielectric material such as silicon oxide layer (SiO₂) or silicon oxynitride (SiON).

Still referring to FIG. 2, the fin 204 includes a channel region (not shown) under the gate stack 208, and source and drain (S/D) regions 216 on opposite sides of the channel region. The gate stack 208 engages the fin 204 in the channel region on three sides of the fin 204. In the S/D regions 216, the FinFET varactor 200 may further include S/D features, such as lightly doped S/D features, heavily doped S/D features, silicide features, and raised S/D features. In the following discussion, unless otherwise noted, S/D regions and S/D features are used interchangeably. As shown in FIG. 2, the S/D regions 216 of the FinFET varactor 200 are connected to a common S/D terminal and the gate stack 208 is connected to a G (gate) terminal, thereby forming a two-terminal capacitor. The S/D terminal and the G terminal correspond to the two terminals of C2 in FIG. 1. In some applications, the FinFET varactor 200 works in an inversion mode, i.e. it works in a range where its capacitance increases as Vg (voltage across the G terminal and the S/D terminal) increases, as exemplarily shown on the right part of the C-V curve 102 of FIG. 1.

The substrate 202 may comprise silicon or another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In an alternative embodiment, the substrate 202 is a semiconductor on insulator (SOI) substrate. The fin 204 may comprise silicon, germanium, or other semiconductor materials.

FIG. 3A shows a cross-sectional view of the FinFET varactor 200 along the A-A line of FIG. 2, in accordance with an embodiment. Referring to FIG. 3A, the FinFET varactor 200 is an NMOS (or n-type) FinFET varactor in this embodiment. The fin 204 includes a well 214, which is a P well in this embodiment. For example, the P well 214 is formed by doping the substrate 202 with a p-type dopant such as boron, indium, or other suitable materials. The FinFET varactor 200 includes n-type doped S/D regions 216 on both sides of the gate stack 208. In an embodiment, the S/D regions 216 are heavily doped with an n-type dopant such as phosphorus, arsenic, or combinations thereof. The FinFET varactor 200 further includes an n-type doped region 218 electrically connecting the S/D regions 216. In a typical NMOS FinFET varactor, the region between the S/D regions 216 is not doped with an n-type dopant, or is dominated by the p-type dopant of the well 214. The present embodiment implements a counter-doping technique to form the region 218, which helps reduce threshold voltage in the FinFET varactor 200.

FIG. 3A further shows doping profiles of the n-type and p-type dopants in the fin 204 along the C-C line of FIG. 3A. Referring to FIG. 3A, curves 222 and 224 illustrate the doping profiles of the p-type dopant and n-type dopant respectively. The dopant level (or dopant concentration) of the p-type dopant peaks in the fin 204 at a depth below the region 218. The dopant level of the n-type dopant peaks in the region 218. Particularly, the dopant level of the n-type dopant exceeds that of the p-type dopant in the region 218, which creates an n-type doping net effect in the region 218 (“counter-doping”). The n-type doped region 218 leads to lowered Vt (threshold voltage) of the NMOS FinFET varactor 200. As a result, the linear tuning range of the FinFET varactor 200 is shifted lower (to the left of the C-V curve) and settles near 0V, as shown in FIG. 1. In an embodiment, the region 218 may be formed at or near 50 nm below the surface of the channel region, and may have a thickness ranging from few nanometers (nm) to few tens of nanometers, such as from 2 nm to 10 nm. Below the region 218, the p-type dopant dominates, creating a p-type doping net effect. Above the region 218 is a region 220 where either the n-type dopant or the p-type dopant may dominate. In the example shown, the p-type dopant level is higher than the n-type dopant level, creating a p-type doping net effect in the region 220. In another embodiment, the n-type dopant level is higher than the p-type dopant level, creating an n-type doping net effect in the region 220. Furthermore, in this embodiment shown in FIG. 3A, the peak of curve 222 is higher than the peak of curve 224. However, this is not limiting. In another embodiment, the peak of curve 222 may be equal to or lower than the peak of curve 224 in the NMOS FinFET varactor 200.

FIG. 3B shows a cross-sectional view of the FinFET varactor 200 along the A-A line of FIG. 2, in accordance with another embodiment. Referring to FIG. 3B, the FinFET varactor 200 is a PMOS FinFET varactor in this embodiment. The fin 204 includes an N well 214. In an example, the N well 214 is formed by doping the substrate 202 with an n-type dopant such as phosphorus, arsenic, or combinations thereof. The FinFET varactor 200 includes p-type doped S/D regions 216 on both sides of the gate stack 208. In an embodiment, the S/D regions 216 are heavily doped with a p-type dopant such as boron or indium. The FinFET varactor 200 further includes a p-type doped region 218 electrically connecting the S/D regions 216. In a typical PMOS FinFET varactor, the region between the S/D regions 216 is not doped with a p-type dopant, or is dominated by the n-type dopant of the well 214. The present embodiment implements a counter-doping technique to form the region 218, which helps reduce threshold voltage in the FinFET varactor 200.

FIG. 3B further shows doping profiles of the n-type and p-type dopants in the fin 204 along the C-C line of FIG. 3B. Referring to FIG. 3B, curves 222 and 224 illustrate the doping profiles of the p-type dopant and n-type dopant respectively. The dopant level of the n-type dopant peaks in the fin 204 at a depth below the region 218. The dopant level of the p-type dopant peaks in the region 218. Particularly, the dopant level of the p-type dopant exceeds that of the n-type dopant in the region 218, which creates a p-type doping net effect in the region 218 (“counter-doping”). The p-type doped region 218 leads to lowered Vt (threshold voltage) of the PMOS FinFET varactor 200. As a result, the linear tuning range of the FinFET varactor 200 is shifted to near 0V, as shown in FIG. 1. In an embodiment, the region 218 may be formed at or near 50 nm below the surface of the channel region, and may have a thickness ranging from few nanometers (nm) to tens of nanometers such as from 2 nm to 10 nm. Below the region 218, the n-type dopant dominates, creating an n-type doping net effect. Above the region 218 is a region 220 where either the n-type dopant or the p-type dopant may dominate. In the example shown, the n-type dopant level is higher than the p-type dopant level, creating an n-type doping net effect in the region 220. In another embodiment, the p-type dopant level is higher than the n-type dopant level, creating a p-type doping net effect in the region 220. Furthermore, in this embodiment shown in FIG. 3B, the peak of curve 224 is higher than the peak of curve 222. However, this is not limiting. In another embodiment, the peak of curve 224 may be equal to or lower than the peak of curve 222 in the PMOS FinFET varactor 200.

FIG. 4 shows a block diagram of a method 400 of forming a FinFET device having a FinFET varactor according to various aspects of the present disclosure. The method 400 is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 400, and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. The method 400 is described below in conjunction with FIGS. 5A-13B which are top or cross-sectional views of a FinFET device 200 according to various aspects of the present disclosure. Specifically, FIGS. 5A, 6A, 6B, 8A, 9A, 10A, 11A, 12A, and 13A are cross-sectional views of the FinFET device 200 along the “A-A” line of FIG. 2; FIGS. 5B, 8B, 9B, 10B, 11B, 12B, and 13B are cross-sectional views of the FinFET device 200 along the “B-B” line of FIG. 2; and FIG. 7 is a top view of the FinFET device 200.

The FinFET device 200 may be an intermediate device fabricated during processing of an integrated circuit (IC), or a portion thereof, that may comprise static random access memory (SRAM) and/or other logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as p-type field effect transistors (PFET), n-type FET (NFET), metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof.

At operation 402, the method 400 (FIG. 4) receives a precursor of the device 200. For the convenience of discussion, the precursor is also referred to as the device 200. Referring to FIG. 5A, the device 200 includes a substrate 202, an isolation feature 206 disposed over the substrate 202, and a fin 204. The fin 204 extends upwardly from the substrate 202 and protrudes out of the isolation feature 206. The substrate 202 is a silicon substrate in the present embodiment. Alternatively, the substrate 202 may comprise another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In yet another alternative, the substrate 202 is a semiconductor on insulator (SOI). The substrate 202 may include buried oxide and strained features.

In some embodiments, the fin 204 is formed from the substrate 202 using suitable processes including photolithography and etching processes. The photolithography processes include forming a photoresist layer (resist) overlying the substrate 202, exposing the resist to a pattern, performing post-exposure bake processes, and developing the resist to form a masking element including the resist. The masking element is then used for etching recesses into the substrate 202, leaving the fin 204 on the substrate 202. The etching process can include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes. Alternatively, the fins 204 may be formed by etching trenches into the isolation feature 206 and epitaxially growing semiconductor materials in the trenches as the fins 204.

The isolation feature 206 may include silicon oxide or other suitable dielectric material. In an embodiment, the isolation feature 206 is formed by depositing one or more dielectric materials over the substrate 202 after the fin 204 is formed, and is then recessed by one or more etching processes. For example, the one or more dielectric materials may be deposited by chemical vapor deposition (CVD) processes, and the etching processes may include dry etching, wet etching, and other suitable etching process.

At operation 404, the method 400 (FIG. 4) forms a well 214 in the fin 204. In an embodiment, operation 404 includes a well ion implantation process which implants one or more p-type dopants 215 (for NMOS FinFET 200) or n-type dopants 215 (for PMOS FinFET 200) into the fin 204 (FIG. 6A). In an embodiment, the well 214 includes an n-type dopant 215, such as phosphorous, arsenic, or other suitable dopants, making it an N well. To further this embodiment, the well ion implantation process may implant the n-type dopant 215 with a doping dose ranging from 1e¹¹ to 1e¹⁴ ion/cm² and an energy level ranging from 10 to 1,000 keV. In another embodiment, the well 214 includes a p-type dopant 215, such as boron, indium, or other suitable dopants, making it a P well. To further this embodiment, the well ion implantation process may implant the p-type dopant 215 with a doping dose ranging from 1e¹¹ to 1e¹⁴ ion/cm² and an energy level ranging from 10 to 1,000 keV. The operation 404 may further include an annealing process at an elevated temperature to activate the dopants and/or remove implantation damage to the fin 204 after the well ion implantation. After the well ion implantation and the optional annealing processes, the well 214 is formed in the fin 204, as illustrated in FIG. 6B.

At operation 406, the method 400 (FIG. 4) forms a mask 230 covering parts of the device 200, but leaving the fin 204 uncovered. Referring to FIG. 7, the mask 230 is disposed over the device 200 with an opening where the fin 204 is exposed. The mask 230 covers various areas of the device 200 so that next operation (operation 408) does not impact those areas. The mask 230 may be a photoresist or a hardmask in some embodiments, and may be formed by one or more photolithography processes.

At operation 408, the method 400 (FIG. 4) performs a channel ion implantation to the fin 204. Referring to FIGS. 8A and 8B, the channel ion implantation introduces a dopant 232 into the fin 204. The dopant 232 is of an opposite type than the dopant(s) 215 existing in the well 214. In an embodiment, the well 214 is a P well (doped with a p-type dopant 215), and the dopant 232 is an n-type dopant such as phosphorus, arsenic, a combination thereof, or another suitable dopant. To further this embodiment, the channel ion implantation process may implant the n-type dopant 232 with a doping dose ranging from 1e¹² to 1e¹⁴ ion/cm² and an energy level ranging from 10 to 50 keV. In another embodiment, the well 214 is an N well (doped with an n-type dopant 215), and the dopant 232 is a p-type dopant such as boron, indium, a combination thereof, or another suitable dopant. To further this embodiment, the channel ion implantation process may implant the p-type dopant 232 with a doping dose ranging from 1e¹² to 1e¹⁴ ion/cm² and an energy level ranging from 10 to 50 keV.

In an embodiment, operation 408 implants the dopant 232 to not only the channel region of the fin 204, but also the S/D regions of the fin 204. Since the S/D regions of the fin 204 will have the same type of dopant as the dopant 232, such implantation is desirable and may be used as lightly doped S/D features. In another embodiment, operation 408 dopes only the channel region of the fin 204 by masking the S/D regions of the fin 204 during the channel ion implantation. The operation 408 may further include an annealing process at an elevated temperature to activate the dopants and/or remove implantation damage to the fin 204 after the channel ion implantation.

After the channel ion implantation process and the optional annealing process, a region 218 is formed in the fin 204 at a depth “d,” as shown in FIGS. 9A and 9B. The depth d is around 50 nm in an embodiment and can be tuned to range from few nanometers to few tens of nanometers in various embodiments. The region 218 is oppositely doped than the well 214, as discussed above. Further, FIG. 9A shows that the region 218 stretches from the source region to the drain region. It is noted that an actual shape of the region 218 may not be as regular as illustrated in FIG. 9A. The mask 230 is removed after the operation 408.

The process conditions for the well ion implantation (FIG. 6A) and the channel ion implantation (FIG. 8A) are tuned such that an appropriate dopant profile, such as the dopant profiles 222 and 224 shown in FIGS. 3A and 3B, is formed in the fin 204. For example, the peak doping level of the dopant 232 is formed at a first depth (e.g., 50 nm) beneath the fin surface for tuning the Vt of the FinFET varactor 200 and for connecting the S/D features 216 (FIGS. 3A and 3B). At the same time, the peak doping level of the dopant 215 is formed at a second depth that is deeper than the first depth. This is generally a result of the doping energy in the well ion implantation being higher than that in the channel ion implantation. Both the doping concentration level and doping depth may be tuned, for example, by changing the doping dose level and/or the doping energy level. For an example, the doping depth generally increases when the doping energy increases. As a particular example, the channel ion implantation uses a doping energy ranging from 10 to 50 keV such that the region 218 is formed at a depth suitable for connecting the S/D features 216. For another example, the doping concentration level generally increases when the doping dose increases. A higher doping concentration of the dopant 232 in the region 218 generally leads to a lower device Vt. The characteristics of the region 218 (e.g., the doping concentration level and the depth) may be designed in conjunction with the work function of the gate 208 (FIGS. 3A and 3B) to obtain a desirable C-V curve of the varactor 200, such as the C-V curve 102 of FIG. 1.

At operation 410, the method 400 (FIG. 4) forms a first gate stack 234 over the fin and above the channel region, as shown in FIGS. 10A and 10B. Referring to FIG. 10A, the gate stack 234 includes multiple layers, such as a gate dielectric layer 236 and a polysilicon (or poly) layer 238. The gate dielectric layer 236 may include a dielectric material such as silicon oxide (SiO₂) or silicon oxynitride (SiON); and may be formed by chemical oxidation, thermal oxidation, atomic layer deposition (ALD), chemical vapor deposition (CVD), and/or other suitable methods. The poly layer 238 may be formed by suitable deposition processes such as low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). The gate stack 234 may further include other layers, such as an interfacial layer, a metal gate layer, and/or other material layers. FIG. 10A further shows a spacer 240 formed on sidewalls of the gate stack 234. The spacer 240 may include one layer or multiple layers, and may contain silicon nitride, silicon oxide, silicon oxynitride, and/or other dielectric materials. The spacer 240 may be formed by a process including a deposition process and an anisotropic etching process.

As discussed earlier, the provided subject matter can be similarly applied to fabricating planar FET devices. In another embodiment, the fin 204 is a planar layer comprising one or more semiconductor materials, and the gate stack 234 is disposed only on a top surface of the planar layer, thereby forming a planar FET instead of a FinFET.

At operation 412, the method 400 (FIG. 4) forms S/D features 216 in or on the S/D regions of the fin 204. Referring to FIGS. 11A and 11B, the S/D features 216 are formed on both sides of the gate stack 234. Particularly, the S/D features 216 are connected through the region 218 in the present embodiment. In an embodiment, the S/D features 216 are formed in the fin 204 by heavily doping the S/D regions thereof. The dopants in the S/D features 216 are of the same type as that in the region 218. For example, they are both n-type dopant for an NMOS FinFET varactor 200, or they are both p-type dopant for a PMOS FinFET varactor 200. However, the dopant concentration in the S/D features 216 is much higher than that in the region 218. For example, the dopant concentration in the S/D features 216 may be on the order of 1e²⁰ cm⁻³ while the dopant concentration in the lightly doped region 218 may be on the order of 1e^(is) cm⁻³. In another embodiment, the S/D features 216 are formed by etching the S/D regions of the fin 204 to form recesses and epitaxially growing a semiconductor material in the recesses. The semiconductor material may be in-situ or ex-situ doped with the proper dopant as discussed above. The S/D features 216 may include other features, such as silicides.

At operation 414, the method 400 (FIG. 4) replaces the first gate stack 234 with a high-k/metal gate (HK/MG) stack 235. Referring to FIGS. 12A and 12B, a dielectric layer 242 is deposited over the device 200, and is planarized (e.g., by a chemical mechanical planarization (CMP) process) to expose the gate stack 234 (FIG. 11A). The dielectric layer 242 may include materials such as tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/or other suitable dielectric materials. The dielectric layer 242 may be deposited by a PECVD process, a flowable CVD process, or other suitable deposition technique.

Subsequently, the various layers of the gate stack 234 are removed by one or more etching processes, thereby forming a trench 241 between opposing walls of the spacer 240. Referring to FIGS. 13A and 13B, various layers are deposited into the trench 241 to form the HK/MG stack 235, including a high-k gate dielectric layer 244, a work function metal layer 246, and a metal electrode layer 248. The HK/MG stack 235 may further include a capping layer, a diffusion barrier layer, and/or an interfacial layer.

In an embodiment, the high-k gate dielectric layer 244 may include a dielectric material such as hafnium oxide (HfO₂), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitable metal-oxides, or combinations thereof. The high-k gate dielectric layer 244 may be formed by ALD and/or other suitable methods.

In an embodiment, the work function metal layer 246 may be an n-type work function metal layer for NMOS FinFET varactor 200 or a p-type work function metal layer for PMOS FinFET varactor 200. The n-type work function layer comprises a metal with sufficiently low effective work function, selected from but not restricted to the group of titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), or combinations thereof. The p-type work function layer comprises a metal with a sufficiently large effective work function, selected from but not restricted to the group of titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof. The work function metal layer 246 may include a plurality of layers and may be deposited by CVD, PVD (physical vapor deposition), and/or other suitable process. In the present embodiment, the work function metal layer 246 is designed to work with the characteristics of the region 218 to tune the C-V curve of the varactor 200. For example, if the well implantation in the operation 404 creates a 0.2V effective threshold voltage in the region 218, and the counter channel ion implantation in the operation 408 creates a −0.4V effective threshold voltage in the region 218, then the region 218 has a net −0.2V effective threshold voltage. In this case, the work function of the gate 234 can be designed to be 0.2V to obtain a net threshold voltage, Vt, at 0V. Alternatively, the work function of the gate 234 can be designed to obtain a net threshold voltage near 0V such as between 0V and 0.3V.

In an embodiment, the metal electrode layer 248 may include aluminum (Al), tungsten (W), cobalt (Co), copper (Cu), and/or other suitable materials. The metal electrode layer 248 may be formed by CVD, PVD, plating, and/or other suitable processes.

At operation 416, the method 400 (FIG. 4) performs further steps to form a final device. For example, the method 400 may form a gate contact electrically connecting the gate stack 235, form S/D contacts electrically connecting the S/D features 216, form metal interconnects to connect the S/D contacts, and form metal interconnects to connect the gate contact (or gate terminal) and the S/D contacts (or S/D terminals) to appropriate circuit nodes to form a complete IC or a part thereof such as the VCO 100 of FIG. 1.

Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a FinFET varactor and the formation thereof. For example, the FinFET varactor according to embodiments of the present disclosure has a low Vt which enables a tuning range of the varactor centered at 0V or at near 0V (e.g., 0.1V). This greatly reduces the power consumption of the circuit where the FinFET varactor is used. Methods of forming the low Vt FinFET varactor according to the present disclosure are fully compliant with existing FinFET manufacturing flow and can be easily integrated therein. Also, the provided subject matter allows the tuning of the C-V curve of the varactor by adjusting channel ion implantation conditions (e.g., dopant species, dose, and energy level) and the work function of the metal gate.

In one exemplary aspect, the present disclosure is directed to a method of forming of a semiconductor device. The method includes receiving a precursor having a semiconductor layer over a substrate, the semiconductor layer having a channel region and source and drain regions on opposite sides of the channel region. The method further includes forming a well in the semiconductor layer, the well having a first dopant. The method further includes implanting a second dopant into the well, the second dopant and the first dopant being of opposite doping types, a first portion of the well having a higher concentration of the second dopant than the first dopant, a second portion of the well under the first portion having a higher concentration of the first dopant than the second dopant. The method further includes forming a gate stack over the channel region; and forming source and drain features in the source and drain regions, wherein the first portion of the well electrically connects the source and drain features.

In another exemplary aspect, the present disclosure is directed to a method of forming a FinFET device. The method includes receiving a precursor having a substrate, an isolation feature over the substrate, and a fin protruding out of the substrate and through the isolation feature, the fin having a channel region and source and drain regions on opposite sides of the channel region. The method further includes implanting a first dopant into the fin, thereby forming a well in the fin; and forming a mask covering areas of the precursor and leaving at least the channel region of the fin uncovered. The method further includes implanting a second dopant into the well through the mask where the second dopant and the first dopant are of opposite doping types, resulting in a first portion of the well having a higher concentration of the second dopant than the first dopant and a second portion of the well under the first portion having a lower concentration of the second dopant than the first dopant. The method further includes forming a first gate stack over the channel region of the fin; forming source and drain features in the source and drain regions of the fin, wherein the first portion of the well conductively connects the source and drain features; and replacing the first gate stack with a high-k/metal gate stack.

In yet another exemplary aspect, the present disclosure is directed to a fin FinFET varactor. The FinFET varactor includes a substrate; a semiconductor fin over the substrate; and a gate stack engaging the semiconductor fin. The semiconductor fin includes a well doped with a first dopant; a channel region under the gate stack; and a source region and a drain region defined in the well and on opposite sides of the channel region. The source and drain regions are doped with a second dopant. A first portion of the well under the gate stack is doped with a third dopant and has a higher concentration of the third dopant than the first dopant. The first dopant is of a first doping type, and the second and third dopants are of a second doping type that is opposite to the first doping type.

The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of forming a semiconductor device, the method comprising: receiving a precursor having a semiconductor layer over a substrate, the semiconductor layer having a channel region and source and drain regions on opposite sides of the channel region; forming a well in the semiconductor layer, the well having a first dopant; implanting a second dopant into the well, the second dopant and the first dopant being of opposite doping types, a first portion of the well having a higher concentration of the second dopant than the first dopant, a second portion of the well under the first portion having a higher concentration of the first dopant than the second dopant; forming a gate stack over the channel region; and forming source and drain features in the source and drain regions, wherein the first portion of the well electrically connects the source and drain features.
 2. The method of claim 1, wherein the semiconductor layer is a fin protruding out of the substrate.
 3. The method of claim 1, wherein the first portion has a thickness ranging from 2 to 10 nanometers.
 4. The method of claim 1, wherein the first portion is formed at a depth of 50 nanometers below a top surface of the semiconductor layer.
 5. The method of claim 1, wherein the implanting of the second dopant into the well includes implanting the second dopant into the channel, source, and drain regions.
 6. The method of claim 1, wherein the forming of the source and drain features includes doping the source and drain regions with the second dopant to have a higher concentration of the second dopant than in the first portion of the well.
 7. The method of claim 1, wherein the forming of the gate stack includes designing a work function of the gate stack to obtain a threshold voltage of 0V.
 8. The method of claim 1, further comprising, before the implanting of the second dopant: forming a mask covering portions of the semiconductor layer and exposing the channel region.
 9. The method of claim 1, wherein the first dopant is of p-type and the second dopant is of n-type.
 10. A method of forming a FinFET device, the method comprising: receiving a precursor having a substrate, an isolation feature over the substrate, and a fin protruding out of the substrate and through the isolation feature, the fin having a channel region and source and drain regions on opposite sides of the channel region; implanting a first dopant into the fin, thereby forming a well in the fin; forming a mask covering areas of the precursor and leaving at least the channel region of the fin uncovered; implanting a second dopant into the well through the mask where the second dopant and the first dopant are of opposite doping types, resulting in a first portion of the well having a higher concentration of the second dopant than the first dopant and a second portion of the well under the first portion having a lower concentration of the second dopant than the first dopant; forming a first gate stack over the channel region of the fin; forming source and drain features in the source and drain regions of the fin, wherein the first portion of the well conductively connects the source and drain features; and replacing the first gate stack with a high-k/metal gate stack.
 11. The method of claim 10, wherein the implanting of the first dopant uses a doping energy higher than that of the implanting of the second dopant.
 12. The method of claim 11, wherein the implanting of the first dopant uses a doping dose higher than that of the implanting of the second dopant.
 13. The method of claim 11, wherein the implanting of the second dopant uses a doping energy ranging from 10 to 50 keV.
 14. The method of claim 10, wherein the replacing of the first gate stack includes selecting a work function of the high-k/metal gate stack to obtain a threshold voltage less than 0.3V.
 15. The method of claim 10, wherein the implanting of the second dopant includes implanting the second dopant into the channel, source, and drain regions of the fin.
 16. A FinFET varactor, comprising: a substrate; a semiconductor fin over the substrate; and a gate stack engaging the semiconductor fin, wherein the semiconductor fin includes: a well doped with a first dopant; a channel region under the gate stack; and a source region and a drain region, defined in the well and on opposite sides of the channel region, the source and drain regions being doped with a second dopant, wherein a first portion of the well under the gate stack is doped with a third dopant and has a higher concentration of the third dopant than the first dopant, wherein the first dopant is of a first doping type, and the second and third dopants are of a second doping type that is opposite to the first doping type.
 17. The FinFET varactor of claim 16, wherein the first portion of the well electrically connects the source and drain regions.
 18. The FinFET varactor of claim 16, wherein another portion of the well under the first portion has a lower concentration of the third dopant than the first dopant.
 19. The FinFET varactor of claim 16, wherein the first dopant is an n-type dopant and the second and third dopants are p-type dopants.
 20. The FinFET varactor of claim 16, wherein the first dopant is a p-type dopant and the second and third dopants are n-type dopants. 